Nucleophilic substitution reaction in polyfluoroaromatics: I. Reactions with secondary amine nucleophiles

Perfluorobenzene, chloropentafluorobenzene and dichlorotetrafluorobenzene were reacted with pyrrolidine, piperidine, morpholine, diethylamine and dipropylamine in aprotic polar solvents such as DMF and HMPA. With perfluorobenzene monosubstituted products were obtained. Prolonging reaction time or raising the reaction temperature caused the formation of the para disubstituted products in DMF. With chloropentafluorobenzene, only the para substituted products were obtained in DMF. Dichlorotetrafluorobenzene was available as an isomeric mixtures of m:o:p = 73:18:9. Except with pyrrolidine, only the m- and o-dichlorotetrafluorobenzene reacted with the secondary amines to form the expected products. This is because of the fact that reaction in polar solvents favors the formation of the products with substituent para to chlorine, but the p-dichlorotetrafluorobenzene has no para fluorine to be attacked. With the most active pyrrolidine however, attack at the position ortho to chlorine occurs. The activity sequence of these secondary amine nucleophiles are attributed to both steric and polar effect. pyrrolidine>piperidine>morpholine>diethylamine>dipropylamine>diisopropylamine All of the products were identified by NMR, IR, GLC and elemental analyses and isomeric ratios were calculated from quantitative NMR measurements.